<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div{
            width: 0;
            height: 0;
            border-color: red transparent transparent transparent;
            border-width: 50px;
            border-style: solid;
        }
    </style>
</head>
<body>
<div></div>
</body>
</html>